1. Field of the Invention
This invention relates to paper making processes, and more particularly, to processes for reducing the total color in effluent streams originating in the production of pulp and paper materials.
2. Description of the Prior Art
Large amounts of water are used in the various stages of the papermaking process. The papermaking process includes several steps, i.e., bark removal, pulping, bleaching, etc. Each of these steps uses a great deal of water. While significant improvements have been made in conserving and reusing water in the papermaking process, it is still necessary to discharge a certain amount of waste water from the system.
The effluent water stream from a pulp mill is contaminated with lignins, lignin degradation products and humic acids. These contaminants make the effluent stream dark colored and are often referred to as color bodies. Since pulp mill plants produce large quantities of this densely-colored effluent, the discharge of this effluent into adjacent streams and bodies of water can cause an objectionable discoloration of the water.
FIG. 1 is a schematic representation of a typical pulp mill plant's waste water treatment system. The effluent stream 1 leaving the pulp plant empties into a settling basin 3. The pH of the effluent entering basin 3 is generally from pH 9 to 10. Various processes have been proposed for the decolorization of the effluent at this stage. Conventional non-biological effluent treatment processes, such as the precipitation of the suspended solids with lime, polyelectrolyte polymers or inorganic metallic salts are effective in removing some color from such effluent. However, polyelectrolyte polymers are prohibitively expensive for treating the quantities of effluents generated by commercial-size pulp and paper plants. Furthermore, inorganic metallic salts produce a great deal of sludge when used at appropriate levels to treat this quantity of water. This sludge must be removed and disposed of at a cost which renders these metallic salts impractical. Furthermore, several inorganic metallic salts will not precipitate suspended solids at a pH of 9 to 10. Therefore, an additional step of lowering the pH of the effluent would be required.
Once the suspended solids have been precipitated in basin 3, the effluent stream moves through channel 5 into bio-degradation basins 7 and 9. The pH of the stream entering the bio-degradation basins 7 and 9 must be between pH 7 and 8 in order to prevent damage to the micro-organisma in the basins. Generally, the pH of the stream entering basins 7 and 9 is lowered to pH 7-8 by injecting carbon dioxide gas into the stream as it travels through channel 5. The cost of the injected carbon dioxide can be as high as several hundred thousand dollars annually in a large pulp mill plant. On the other hand, if, however, the pH of the effluent in the basin 3 has been lowered below pH 7 so that metallic salts, such as ferric iron salts, can be used as the precipitating agent, the pH would have to be raised back up to 7 or 8. Moreover, if inorganic metallic salts, such as ferric iron salts, are used to precipitate the suspended solids in the basin 3, these metallic salts must be removed prior to the effluent entering the bio-degradation basins 7 and 9, since high concentrations of metallic ions are toxic to the micro-organisms in the basins 7 and 9. The effluent thereafter leaves the bio-degradation basins 7-9 via channel 11 into setting pools 13 and 15, and is subsequently discharged into a lake, stream or municipal sewer.
In view of the deficiencies of the prior processes for decolorization of pulp mill effuents, it is an object of this invention to provide an efficient and practical process for decolorization of effluents.
It is a further object of the invention to provide an efficient process for decolorization of pulp mill effluents in which the discharge stream is of adequate quality to be conducted into natural bodies of water without further treatment.